1. Field of the Invention
The present invention relates to a cooling apparatus of an electronic device e.g. a semiconductor chip, a chip module containing a single semiconductor chip and a multichip module containing multiple semiconductor chips.
2. Description of the Related Art
Japanese Patent Application Laid-open No. 62-268148 disclose a prior-art device for cooling a semiconductor chip by contact through a thermal conductive material with a cooling solid body.
U.S. Pat. No. 4,567,505 as illustrated in FIG. 1 discloses a prior-art arrangement in which a liquid e.g. silicon oil, readily wetting and spreading over the surface of a solid body is sandwiched between a heat transfer surface of a cooling solid body, which surface defines multiple microgrooves in the form of a so-called re-entrant structure with a narrower entrance and a divergent portion extending to the bottom of each of the microgrooves and a flat heat transfer surface of a semiconductor chip constituting a heating element, so that the surface tension of the liquid brings said two heat transfer surfaces into close contact with each other so as to reduce a contact thermal resistance therebetween.
The former publications fails to teach that the thickness of a layer of the thermal conductive material sandwiched between the semiconductor chip and the cooling solid body is reduced in order to increase the thermal conductance of the layer or to provide any suggestion of an electronic device in the form of a multichip module containing multiple semiconductor chips.
In the case of the cooling structure disclosed in U.S. Pat. No 4,567,505, only the surface tension of the liquid draws the cooling solid body to the semiconductor device, so that when the heat transfer surface of either of the cooling solid body or the semiconductor device has a warp, a rigidness of the cooling solid body or the semiconductor device prevents any possiblity of rectifying the warp by means of the draw force of the liquid and thus the thickness of the liquid layer between said two heat transfer surfaces cannot be maintained constant. That is, since there is no external force restraining a deformation in the heat transfer surfaces, the thickness of the liquid layer is changed due to the magnitude of the warp in the contacted heat transfer surfaces. Thus, the contact thermal resistance between the contacted heat transfer surfaces will not be kept constant.
On the other hand, when the cooling solid body which has been drawn to the semiconductor device is going to be separated from the semiconductor, the liquid is drawn to the entrance of the re-entrant groove and the radius of curvature of the concave surface of the liquid is much reduced as the cooling solid body separates from the semiconductor. Consequently, the pressure of the liquid decreases to below the surrounding pressure and the draw force of the liquid increases. Thereby, a required separation force must increase. If the separation force is reduced, then the draw force of the liquid will decrease, so that the contact thermal resistance will not be reduced. On the other hand, if the draw force of the liquid is increased, then the semiconductor device itself and/or an electric connection to the semiconductor device will experience damage when the cooling solid body is separated from the semiconductor device.
As described above, the prior art fails to teach that a bearing pressure between the contact surfaces is maintained low and a cooling power is concurrently increased, and a separation of the cooling solid body from an object to be cooled concurrently is easy.